Field of the Disclosure
The present disclosure is directed to a method to prepare an electrolyte for a magnesium battery.
Discussion of the Background
Magnesium batteries have been the subject of high interest and significant research and development effort in order to provide more economical, safer and higher capacity batteries to displace or supplement the conventional lithium batteries. Compared to lithium, Mg potentially has a volumetric capacity of 3832 mAh cm−3 which is significantly greater than the 2062 mAh cm−3 of Li. Additionally, Mg has a negative reduction potential of −2.356V vs NHE. As the seventh most abundant element in the earth's crust, Mg has a lower resource cost and a lower environmental impact profile.
An ongoing objective in battery research is increasing the energy density beyond that offered by lithium ion batteries. This may require a shift towards batteries containing a pure metal anode. However, in the case of lithium, deposition occurs unevenly with formation of dendrites which leads to safety concerns during cycling and renders Li metal unsafe for commercialization as a high capacity anode material. In contrast to lithium metal, magnesium metal deposition is not plagued by dendritic formation. Additionally, magnesium is more stable than lithium when exposed to air. However, magnesium has a reductive potential of −2.356 V vs. NHE and has a unique electrochemistry which precludes the use of magnesium electrolytes that are analogues of lithium electrolytes. Reduction of magnesium analogues such as Mg(PF6)2, Mg(ClO4)2 and Mg(TFSI)2 results in the formation of a blocking film on the magnesium anode surface through which successful deposition of magnesium has not been observed (Feng, Z: Surface Coating Technologies, Vol 201, pp 3783-3787, 2006).
Reports of effective magnesium electrodeposition from Grignard reagents in ethereal solutions date as far back as 1927 and have periodically appeared in the literature ever since. In an attempt to enhance the stability of the electroplating baths based on Grignards, in 1957 Connor et al. (Connor: Journal of The Electrochemical Society, Vol 104, pp 38-41, 1957) investigated the electrodeposition of magnesium from magnesium borohydride Mg(BH4)2 generated in situ by the reaction of MgBr2 and LiBH4. Unfortunately, boron and magnesium co-deposit in a 1:9 ratio. Recently, Mohtadi et al. (Mohtadi: Angewandte Chemie International Edition, Vol 51, pp 9780-9783, 2012) have demonstrated the use of magnesium borohydride as an electrolyte for magnesium battery. The oxidative stability of Mg(BH4)2 has been reported similar to Grignard solutions. However, one of the obstacles in developing high voltage rechargeable magnesium batteries is moving beyond the oxidative stability of Grignard reagents such as ethylmagnesium bromide (EtMgBr) and butylmagnesium chloride (BuMgCl) which have an oxidative stability of 1.3V vs. Mg. The low oxidative stability of these Grignard solutions limits the choice of available cathodes. In 1990, Gregory et al. (Gregory: Journal of The Electrochemical Society, Vol 137, pp 775-780, 1990) synthesized an electrolyte Mg[B(C4H9)4]2 from the reaction of dibutylmagnesium and the Lewis acid tri-n-butylborane which showed enhanced oxidative stability versus BuMgBr. It was assumed that the character of the Lewis acid could be a factor in improving the voltage stability. Gregory also evaluated magnesium deposit quality by spiking of alkyl Grignards such as ethylmagnesium chloride (EtMgCl) and methylmagnesium chloride (MeMgCl) with aluminum trichloride (AlCl3) to enhance electrochemical plating.
Aurbach et al. (Aurbach: Nature, Vol 407, pp 724-727, 2000) has popularized a novel class of electrolytes called magnesium organohaloaluminates. One such electrolyte called APC is generated in situ by the reaction of aluminum trichloride (AlCl3) with the Grignard reagent phenylmagnesium chloride (PhMgCl) in a 1:2 ratio and has an oxidative stability in excess of 3.2 V vs. Mg and can deposit/dissolve magnesium with high coulombic efficiencies. Magnesium organohaloaluminate electrolytes possess a high oxidative stability on inert electrodes (above 3.0 V vs. Mg) such as Pt or glassy carbon and are capable of depositing and stripping magnesium at high currents. However, they have been reported to be corrosive towards less noble metals such as aluminum, nickel and stainless steel which limits charging in a coin cell battery configuration to under 2.2 V due to the utilization of such metals in the casing and current collector material. Since the oxidative stability of electrolytes governs the choice of cathodes, it is of paramount importance to develop a non-corrosive magnesium electrolyte which will permit discovery of high voltage cathodes. Improving the voltage stability of magnesium electrolytes on stainless steel is crucial because stainless steel is a widely used current collector and a major component in a variety of batteries such as coin cells. Current state of the art magnesium organohaloaluminate electrolytes limit the usage of Mg battery coin cells to operating under 2.3V vs Mg.
The present inventors (U.S. patent application Ser. No. 14/263,392) described an ion-exchange pathway that converts a magnesium electrolyte containing chlorides into a chloride-free magnesium borate salt. However, this reference does not describe a one step synthesis of non-chlorinated magnesium electrolytes by reacting a Grignard reagent with a fluorinated aryl borane.
Therefore, an object of the present disclosure is to provide an efficient facile method to prepare a non-chlorinated electrolyte or an electrolyte with low chloride content suitable for a magnesium battery.
It is a further object to prepare non-corrosive or low-corrosive electrolytes for a magnesium battery.
It is a further object to provide magnesium electrochemical cells employing the magnesium electrolyte and magnesium batteries containing the electrochemical cell.